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FATIGUE COUNTS

FATIGUE COUNTS. Rainflow, zero-crossing, bandwidth, etc. RAINFLOW COUNTS. The on-going study: 1. West-Africa spectra are different from those that we are used to. We need first to check our analysis methods in such cases.

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FATIGUE COUNTS

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  1. FATIGUE COUNTS Rainflow, zero-crossing, bandwidth, etc. Michel Olagnon

  2. RAINFLOW COUNTS The on-going study: 1. West-Africa spectra are different from those that we are used to. We need first to check our analysis methods in such cases. 2. It seems impossible to calculate through all combinations of the values of a dozen parameters describing sea states. We need ways to combine the damages created by the components of the sea states at the final stage only. 3. How do we select the calculation cases from the climate description and from the combination formulas ? Michel Olagnon

  3. RAINFLOW COUNTS • Time domain analysis in a composite signal context (Hélène Pineau - Actimar) • Spectral domain - Simple summation and alternatives (Michel Olagnon - Ifremer) • Preliminary results with combination formulas (Zakoua Guédé - Ifremer) Michel Olagnon

  4. Michel Olagnon

  5. With respect to counting all transitions, zero-crossing removes most of the small ranges, and rainflow turns medium-small ranges into smaller ones and, as we will see better on the next figure, into enlargement of the largest ones. Michel Olagnon

  6. Zero-crossing finds the same large ranges as all transitions, whereas they are larger with rainflow: rainflow associates high peaks to low valleys and smaller peaks to less deep valleys. -> Hélène Michel Olagnon

  7. Simple Summation: add independent damages... …yet: 10 + 1 = 11 cycles range 2, m=4 -> damage ~ 176 9 cycles range 1.78 in average, 1 cycle range 4 m=4 -> damage ~ 346 Michel Olagnon

  8. Simple summation is not conservative, yet it would be nice to still be able to compute separately damage for each component and to have a (non-linear) way to make the combination. Michel Olagnon

  9. RAINFLOW COUNTS Separate the turning points into two sets: * The maximum (and resp. minimum) over each interval where the low frequency signal is positive (resp. negative) * The other turning points Michel Olagnon

  10. RAINFLOW COUNTS The Simple Summation formula D = DH + DL The Combined Spectrum formula D = NH+L((DH/NH)2/m + (DL/NL)2/m)m/2 The DNB formula D = (NH-NL)/NH DH + NL((DH/NH)1/m + (DL/NL)1/m)m The new formulae D = (NH-NL)/NH DH + Z(L/H+L ,NL/NH, m) DL The CNB (Jiao-Moan) formula D = F1(L/H+L ,NL/NH, m) DH + F2(L/H+L ,NL/NH, , m) DL The “exact” formula D = WAFO (Combined Spectrum) Michel Olagnon

  11. RAINFLOW COUNTS The new formulae D = (NH-NL)/NH DH + F(, , m) DL • ML distributed as the maximum of NH/Nl maxima of the global signal : D = (1-)DH + Z1(m, ) ((1-2)/(1-2))m/2 DL • ML distributed as the highest NH/Nl maxima of the global signal :D = (1-)DH + Z2(m, ) ((1-2)/(1-2))m/2 DL • ML distributed as the highest NH/Nl narrow-band maxima of the global signal:D = (1-)DH + 1/ Q(m/2+1, -Ln()) / (1-2)m/2 DL • ML distributed as the sum of two Rayleigh variables (Igor). • DDNB = (1-)DH + (1+  /(1-2))m DL -> Zakoua Michel Olagnon

  12. RAINFLOW COUNTS What does accuracy mean ? Michel Olagnon

  13. RAINFLOW COUNTS Sensitivityto spectral bandwidth secondary wrt aleatory uncertainty. Michel Olagnon

  14. RAINFLOW COUNTS How much conservatism is needed ? How much is too much ? 92% Michel Olagnon

  15. West Africa Spectra -> Wide band wave action Mooring responses -> Composite signals What does wide-band mean ? Michel Olagnon

  16. Is this a wideband spectrum ? If yes, is it because of the ears or of the tail ? Michel Olagnon

  17. Which is the widest ? Michel Olagnon

  18.  = 0.776  = 0.604 Michel Olagnon

  19. Effect on the rainflow ranges distribution when removing the spectrum tail: very similar to removing the smallest cycles. Michel Olagnon

  20. What kind of S-N curve slopes are we designing for ? Michel Olagnon

  21. Do we need double slopes ? Can we use thresholds ? Michel Olagnon

  22. Are the components independent ? Michel Olagnon

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